1. Field of the Invention
The invention relates to a legged robot and a control method of the legged robot.
2. Description of the Related Art
To ensure that a legged mobile robot moves with stability and without falling over, a supporting leg of the robot must be brought into close contact with a road surface. It is also desirable to suppress unforeseen outside disturbance forces from road surface irregularities when a lifted leg of the robot is grounded. It is therefore important to control the relationship between the soles of the robot and the road surface in accordance with a target.
In related art, many techniques of disposing a sensor on the soles of the robot and aligning an output value of the sensor with a target value have been proposed. Japanese Patent No. 3574952 discloses a bipedal movement device and a walking control device for the device that detects a floor reaction force on the sole of the foot, thereby realizing walking stability even on an unstable road surface having complicated irregularities. In the bipedal movement device described in Japanese Patent No. 3574952, three sensors are selected from among at least three triaxial force sensors disposed on the sole in descending order of the force detected thereby, and the gait is corrected based on corresponding floor reaction force data.
Japanese Patent No. 3569768 discloses a bipedal movement device with which a foot portion can be grounded securely even on an unstable road surface having complicated irregularities. In the bipedal movement device described in Japanese Patent No. 3569768, a triaxial force sensor is provided on a heel portion and a toe portion, and based on signals from these force sensors, gait data are corrected such that three-point support is achieved by the heel portion and the toe portion.
Japanese Patent Application Publication No. 2001-353686 (JP-A-2001-353686) discloses a foot portion structure for a legged mobile robot and a road surface detection apparatus for accurately determining the condition of a road surface and the grounding condition of a foot portion while the legged mobile robot is walking. The foot portion structure for a legged mobile robot described in JP-A-2001-353686 includes a foot portion base body and a road surface detection unit disposed on the bottom face of the foot portion base body to measure the approach of the foot portion to the road surface and the grounding condition of the foot portion. A road surface detector is attached to the foot portion base body to be free to emerge therefrom, and a road surface detection unit detects the emergence position of the road surface detector. JP-A-2001-353686 also discloses means for achieving stability by feedback-controlling the distance between the sole and the road surface using a sole distance sensor.
However, with the techniques described in Japanese Patent No. 3574952 and Japanese Patent No. 3569768, it may be impossible to obtain the floor reaction force accurately when more than three sensors come into contact with the road surface. In this case, it may be impossible for all of the sensors to contact the road surface, depending on the road surface condition, leading to an increase in the number of triaxial force sensors that cannot be used to calculate the floor reaction force, and as a result, the floor reaction force cannot be detected accurately. Further, when three sensors are used, an actual convex closure foamed by a contact part between the sole and the road surface decreases in size, leading to a reduction in the size of a stable region in which a Zero Moment Point (to be referred to simply as “ZMP” hereafter) can exist, and as a result, the robot is more likely to fall over. The robot is also more likely to slide in a yaw direction. Hence, providing more than three sensors leads to redundancy, while three sensors are insufficient. For practical purposes, therefore, only a case in which four sensors are provided, as shown in the embodiments, is effective.
Furthermore, with the technique described in JP-A-2001-353686, the distance between the road surface and the sole in the sensor position cannot physically be set at zero when a part other than the sensor contacts the road surface. In other words, it may be impossible to set a measurement height value of all of the distance sensors at zero, depending on the road surface condition.
When controlling the distance relationship between the sole of the robot and the road surface in accordance with a target, the distance sensor is preferably disposed as close to the outer periphery of the sole as possible in order to improve stability. Further, distance sensors are preferably disposed in the four corners to increase the size of the convex closure of the contact portion between the sole and the road surface to a maximum. However, considering that a plane is determined by three points, one contact location between the road surface and the sensor becomes redundant if distance sensors are disposed in the four corners. If the number of distance sensors is redundant, the values of all of the distance sensors cannot be set at desired values when the sole of the robot steps on a complicated irregularity. As shown in FIG. 18, for example, when a part of a foot portion 26 of the robot steps on an obstacle B (i.e. when a point 16a on the rear surface of the foot portion 26 is positioned on the obstacle B), points 16c and 16a of the sole are able to contact the road surface, but points 16b and 16d cannot contact the road surface simultaneously. In other words, when the points 16c and 16a of the sole contact the road surface, the sole rotates about an axis C located on a diagonal passing through the points 16a and 16c, leading to instability. Hence, it may be impossible to bring all four points on the sole of the supporting leg into close contact with the road surface, depending on the irregularity condition of the road surface. When control is performed in this case to align the values of all of the sole distance sensors with desired values, for example when the sole is corrected to minimize a square error of the distance, control may be performed to an extremely unstable condition, for example such that the sole lands on the road surface on the diagonal. In other words, when an attempt is made to align the values of all of the distance sensors with target values, an unstable condition may conversely be maintained. Hence, when the trunk of the robot tilts while the sole is not in close contact with the road surface, the robot is unable to obtain a floor reaction force moment for correcting the tilt and may fall over as a result. To prevent the robot from falling over, it is necessary to choose the distances that are to be aligned with the target values from among the distances of the sole distance sensors provided on the supporting leg.
The floor reaction force moment required to correct the tilted trunk can be obtained by appropriately selecting the sole distance sensors to be aligned with a target height. In a constitution where only three sole distance sensors are selected, it may be impossible to bring the sole of the supporting leg into close contact with the road surface depending on the falling direction of the trunk, leading to instability. As shown in FIG. 19, for example, when one distance sensor 16d steps on a projection on the road surface such that three sole distance sensors 16a, 16b, 16c are selected from a detected falling direction D, the sole distance sensors 16a and 16c cannot contact the road surface simultaneously, and therefore the sole of the supporting leg cannot be aligned completely. The reason for this is that when the tilt direction of the trunk of the robot and the direction of an irregularity on the road surface are close, control is performed such that the sole lands on the road surface on a diagonal, similarly to the case described above, leading to an unstable condition.
Hence, according to the legged robots in the related art, because it is difficult to maintain contact between the sole and the road surface while effectively obtaining a floor reaction force moment for causing the trunk of the robot to an inverted pendulum state, the robot might not walk with stability and without falling over depending on the road surface condition.